During the molding process, a solid article is produced from thermoset resins such as SMC or BMC compositions based on unsaturated polyesters or vinyl esters. These articles often adhere to mold surfaces thereby interfering with article detachment from the mold and slowing mold throughput. The problem is particularly acute for SMC and BMC materials that have low viscosity formulations containing liquid monomers and flowing fibers and other fillers that are allowed to flow into contact with and, in some instances, wet shape determining surfaces of a mold then to build viscosity according to controlled kinetic schedules. With this intimate contact encouraging adhesion of the resultant molded article to the mold surfaces.
To promote the molding process, manufacturers often resort to a mold release agent that facilitates the release of the molded article from the mold. Internal mold release agents such as magnesium stearate and stearic acid are often included in a sheet molding composition (SMC) or bulk molding composition (BMC) to promote release of the molded article from the mold. However, such release agents tend to be uniformly distributed throughout the molding composition and as a result pulling a small percentage of the total mold release composition internal to the SMC or BMC actually functions in a mold release capacity. Adding additional quantities of internal mold release composition to the thermosetting composition to improve mold release comes at the cost of other physical properties of the resultant molded article.
Another general class of mold release agents is external mold release agents that are applied directly to mold surfaces that come into contact with molding composition. The procedure of applying an external mold release to the mold surfaces tends to slow throughput since the mold release must be repeatedly applied to the mold surfaces between individual article moldings. Additionally, the mold release agents tend to build up on the mold surfaces thereby negatively impacting the tolerances of the molded articles and as such, buildup of the mold release agent on the mold surface must be periodically removed there from, resulting in still further costly and time consuming delays to mold throughput.
Conventionally, silicone compositions have been used as mold release agents and have suffered considerable difficulties that have limited their usefulness. Specifically, uncured silicones such as polysiloxane waxes and fluids tend to transfer to the resultant molded article during the de-molding process and require frequent reapplication to the mold. Additionally, this transfer of silicone materials to the molded article negatively impacts the ability to further process the molded article by interfering with paint and other over-layer coatings. Even upon cure, silicone compositions tend to have poor abrasions resistance and are readily abraded and removed from the mold surface by the SMC or BMC molding composition flowing into the mold and containing abrasive materials such as chopped glass fiber and inorganic filler particulate. Additionally, thermoset resin cure inhibitors and catalysts can chemically interact with the mold release agent and degrade the same.
Thus, there exists a need for an external mold release for SMC and BMC formulations that overcomes the problems associated with silicone mold releases. There further exists a need for a process to use such a mold release to increase molding throughput of SMC and BMC thermoset articles.